Code cleanup with more careful error checking before using data read from the commit-graph file. * ab/commit-graph-fixes: commit-graph: improve & i18n error messages commit-graph write: don't die if the existing graph is corrupt commit-graph verify: detect inability to read the graph commit-graph: don't pass filename to load_commit_graph_one_fd_st() commit-graph: don't early exit(1) on e.g. "git status" commit-graph: fix segfault on e.g. "git status" commit-graph tests: test a graph that's too small commit-graph tests: split up corrupt_graph_and_verify()